The present invention relates in general to optical devices, and pertains, more particularly, to optical switching and/or bistable devices. Even more particularly, the present invention is concerned with a novel technique by which one can change the dispersion relation of surface plasmons at a metal-non-linear dielectric interface by varying the input laser intensity incident upon the device.
Optical bistability in reflection has been examined recently with considerable interest. See, for example, the article entitled "Optical Bistability With Surface Plasmons" by G. M. Wysin, H. J. Simon, and R. T. Deck from Optics Letters, Volume 6, No. 1, pages 30-32, January, 1981. The theory of hysteresis reflection by the boundary of a non-linear medium at grazing-angle incidence was first proposed by Kaplan in the article "Theory of Hysteresis Reflection and Refraction of Light by a Boundary of a Non-Linear Medium" by A. E. Kaplan, Sov. Phys., JEPT 45, 896 (1977). In the article "Bistability at a Non-Linear Interface" by P. W. Smith, et al, Applied Physics Letters, 35, 846 (1979), there has been demonstrated optical bistability in reflection at the boundary between a linear glass medium and CS.sub.2, a non-linear (Kerr effect) medium. Herein, when reference is made to a surface-plasmon mode, this refers to an evanescent transverse-magnetic (TM) wave that propagates along an interface between a medium with a negative dielectric constant, such as a metal, and a medium with a positive dielectric constant.
In the aforementioned article, "Optical Bistability With Surface Plasmons" by G. M. Wysin, et al, the bistable device is fabricated by considering an interface between a metal and Kerr non-linear medium with a positive-intensity-dependent index of refraction. The surface-plasmon mode is excited in the Kretschmann configuration with the use of a prism along with a metallic layer which may be a silver film, and a dielectric layer which is a non-linear dielectric. In this same connection, also refer to three papers presented at the 11th International Quantum Electronics Conference in Boston, Jun. 23-26, 1980, papers T8, T9 and T10, pages 657-659. The papers cited are Smith, et al, "Optical Stability and Switching at a Non-Linear Interface", (T8); Smith, et al, "Waveguide Non-Linear Interface Device"--(T9); and Kaplan, "Bistable Reflection of Light From the Boundary of an Artificial Non-Linear Medium"--(T10).
Although, from a theoretical standpoint, the prism may be convenient to use in the aforementioned Kretschmann configuration for surface-plasmon excitation, from a practical standpoint, the prism is not advantageous in actual use in that there is a requirement that the prism be of a different material than both the metal and the dielectric materials and has to have a relatively large index of refraction. This provides an extreme limitation on the prism material and makes it difficult from a practical standpoint to find desirable materials that can be used. Also, in the prism-metal-dielectric arrangement, there are three separate materials that are used and, particularly for strong light intensities, this makes precise control more difficult because of the use of three materials rather than two.
Accordingly, it is an object of the present invention to provide an optical bistable device with surface plasmons in which the device is fabricated primarily only of two materials which include the metal film and non-linear dielectric.
To accomplish the foregoing and other objects of the invention, there is provided an optical bistable device that comprises a non-linear optical material that forms a non-linear dielectric layer. For example, this dielectric layer may be of silicon or gallium arsenide. The optical device also includes an optically thick metal layer and an interface between the metal and non-linear dielectric layers including a sub-micron grating. This sub-micron grating is typically fabricated on the semiconductor surface and the metal layer is then deposited thereover. Means are provided for optically exciting the device from a light source of given intensity. The device operates on the principle of variation of the dispersion relation of surface plasmons as a function of incident intensity and is characterized by hysteresis type bistable operation.